powerpc: booke: Fix build failures

arch/powerpc/platforms/wsp/wsp.c: In function ‘wsp_probe_devices’:
arch/powerpc/platforms/wsp/wsp.c:76:3: error: implicit declaration of function ‘of_address_to_resource’ [-Werror=implicit-function-declaration]

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>